1. Field of the Invention
The invention relates to organopolysiloxane compositions, to their preparation and to their use in low-modulus compositions which can be crosslinked at room temperature, in particular, compositions which crosslink with elimination of alcohols.
2. Background Art
Single-component silicone rubber mixtures (RTV-1) which can be stored in the absence of water, but which can be vulcanized at room temperature when exposed to water to form elastomers are known. These products are used in large amounts, for example as joint sealants in the building industry. These RTV-1 mixtures are based on polydiorganosiloxanes which are terminated either by OH groups or by silyl groups bearing hydrolyzable groups. Significant properties of the RTV-1 mixtures can be influenced via the chain length of the polymers. In particular the modulus, i.e. the stress at 100% elongation of a cured test specimen, can be regulated via the chain length. In the case of joint sealants in particular, it is desirable for the modulus to be low, so that only very low forces can act on the sides of the joint. However, only a restricted range of polymer chain lengths is available for the production of RTV-1 mixtures for economic and technical reasons. In particular, the large chain lengths required for very low-modulus sealants lead to very high viscosities of the polymers, so that such products can be handled only with difficulty. It is therefore desirable to produce low modulus elastomers from easily processable polymers having relatively short chain lengths and therefore relatively low viscosities for the preparation of RTV compositions.
To increase the chain length and hence also the viscosity of the polysiloxanes and thus to decrease the modulus of RTV-1 elastomers produced therefrom, relatively long polymers can be produced from shorter polymers by chain extension. It is known that bifunctional silanes or siloxanes of sufficiently high reactivity can be used for this purpose. Thus, U.S. Pat. No. 5,110,967 describes Si—N-heterocyclic silanes. However, such silanes require specific crosslinkers in the formulation of RTV compositions. Compounds such as bisacetamidosilane (cf., for example, U.S. Pat. No. 5,290,826), bisacetoxysilane (cf., for example, DE-A 12 95 834) or bisaminosilane (cf., for example, EP-A 74 001) liberate elimination products which pose health concerns or are corrosive during vulcanization. For this reason, an alcohol is very frequently preferred as an elimination product during condensation, but dialkoxysilanes and dialkoxysiloxanes such as those described in U.S. Pat. Nos. 5,300,612 and 5,470,934 are unsuitable for a rapid reaction with silanol-terminated siloxanes. The same applies to alkoxysilanes having two alkoxy groups, which do not exhibit chain extension, and which are described in DE-A 198 55 619.
Furthermore, the use of α-aminomethyldialkoxymethylsilanes for chain extension, for example, N,N-dibutylaminomethylmethyldiethoxysilane, is known. Reference may be made, for example, to JP-A 63083167. Although these silanes initially undergo a rapid reaction with the hydroxy-terminated polysiloxanes, the resulting polymer is degraded again in the presence of substances containing active hydrogen, e.g. alcohols, which are always present when the polymers are used in RTV compositions, leading either to the chain extension being reversed and thus made ineffective, or to the polymer being damaged to such an extent that the RTV-1 compositions no longer vulcanize.
A further problem encountered with RTV-1 compositions is their stability over a prolonged period of storage. In the case of RTV-1 compositions which cure by elimination of alcohols, it is particularly difficult to obtain storage-stable products. A number of approaches have been tried to solve the problem. One approach is to remove all OH-containing compounds from the mixtures by means of scavengers, examples of which are isocyanates (cf. U.S. Pat. No. 4,495,331). The isocyanates react with the troublesome OH-containing components to form urethanes which do not adversely affect the storage stability. However, it is necessary for the isocyanates to be used in an effective amount, i.e. at least the amount which is stoichiometrically required for reaction with all OH-containing compounds has to be added. Since this is normally not known precisely for industrial mixtures, a significant excess of scavenger is used in order to be certain that all OH-containing components are indeed reacted.
A further method of increasing storage stability is to add stabilizing components, for example phosphorus compounds, which form complexes with the organotin compounds normally used as curing catalysts (cf. DE-A 101 21 514).